Contamination of closed water areas including lakes and marshes and inland seas by phosphorus and eutrophicated substances has now become a big problem not only in Japan but also all over the world. It is well known that the progress of such contamination is derived from the eutrophicated substances contained in the agricultural effluent, domestic waste water, etc., which flow into the lakes and marshes and inland seas and the lowering of the water level of the water area due to the development of coasts.
Accordingly, techniques for restraining eutrophicated substances from flowing into closed water areas, for example, concentrated treatment of city life type domestic waste water, agricultural community waste water and top soil eluting substances produced by agricultural effluent and rain water, and various techniques for maintaining and enhancing the self purification capacity, for example, techniques of removing deposited sludges and algae, charging chemical substances such as lime and blowing oxygen or air into the water have been carried out.
As a method of softening hard water, a method of depositing alkali earth metals (e.g., CaO) by storing the hard water for a certain period, removing the alkali earth metals by distillation or with an ion exchange resin, and a method of depositing these metals by boiling and using the supernatant liquid are known.
These prior arts, however, involve the following problems: in the case of using an absorbent, flocculating agent, neutralizing agent, oxidizing agent, reducing agent, etc., since secondary contaminating substances are mixed into the treatment water and a large amount of sludge is generated, it is necessary to remove them; and in the case of carrying out other techniques, even if a large sum of equipment cost and running cost is invested, satisfactory effects have not yet been produced.
In a method of storing hard water for a certain period for softeining it in the prior art, if the concentration of the alkali earth metals dissolved in the water is high, the pH of the water becomes weakly alkaline in most cases. Therefore, the water is allowed to stand for a long time to generate and deposit calcium hydroxide or the like, but the behaviour is unstable and, further, with the reduction in the concentration, the amount of deposit produced becomes smaller. On the other hand, in the case of softening water by distillation, with an ion exchange resin, or by boiling, the cost is raised, so that the treatment of a large amount of water is difficult.